1. Field of the Invention
This invention relates to a ferroelectric storage device, more particularly, elongation in the life of the ferroelectric storage device.
2. Description of the Earlier Technology
Ferroelectric memories having ferroelectric capacitors are known as nonvolatile semiconductor memories. FIG. 12 shows a part of a conventional ferroelectric memory in a circuit diagram. The conventional ferroelectric memories comprise a ferroelectric capacitor 4 and a capacitor 6 for load. FIG. 13 is a hysteresis curve showing a relationship between voltage and polarity state in the ferroelectric capacitor 4, wherein the voltage means the potential of a bit line BL when voltage of a plate line PL1 shown FIG. 12 is used as a reference potential and the quantity of electric charge is used to show the level of the polarity state.
In FIG. 13, a first polarity state P1 means a state wherein the capacitor 4 has remanence Z1 and a second polarity state P2 means a state wherein the capacitor 4 has remanence Z2, the remanence Z1 corresponding to data "H" and the remanence Z2 corresponding to data "L". Data can be read out of the ferroelectric capacitor 4 by examining which of these polarity states the capacitor 4 has.
To examine which of these polarity states the ferroelectric capacitor 4 has, a voltage Vf is measured which occurs in both ends of the ferroelectric capacitor 4 when a read-out voltage Vp is applied to the plate line PL, after electricity is discharged from the capacitor 6 for a load shown at FIG. 12 and then the bit line BL is made into a floating state.
According to the graphical solution in FIG. 13, when electrostatic capacity of the capacitor 6 for load respects the slope of the line L1 the voltage Vf occurring in both ends of the ferroelectric capacitor 4 is "V1" if the ferroelectric capacitor 4 has the first polarity state P1 and the voltage Vf is "V2" if the second polarity state P2. Therefore, when a reference voltage is predetermined as shown in FIG. 13 it can be examined which of these polarity states the ferroelectric capacitor 4 has by making a comparison between the voltage Vf occurring in read-out operation and the reference voltage Vref.
Data associated with the polarity state can be read out by means of examining the polarity state of the ferroelectric capacitor 4 in this way.
Such a conventional ferroelectric memory as described above, however, has the following problems. There is a bad electric property (which is referred to as "imprint effect") in ferroelectric capacitors wherein the hysteresis curve distorts when the same polarity state is held for long time.
As a result of that, when a long time passes with the same data being stored in the ferroelectric capacitor 4 this ferroelectric capacitor gets the bad electric property. On getting the bad electric property, there changes the voltage Vf occurring in the both ends of the described-above ferroelectric capacitor 4. In particular, when the opposite data has been written into the ferroelectric capacitor 4 having the bad electric property the opposite data can not be read out correctly. That is, as time passes the function of ferroelectric memory storage deteriorates.